This application is related to Japanese Patent Application No. 2001-157148 filed in May 25, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and particularly to a magneto-optical recording medium which can perform magnetically induced super resolution reproduction.
2. Description of the Related Art
In order to increase recording density, a magneto-optical recording medium has been developed which is provided with a recording mark having a mark length shorter than a spot diameter of a laser beam and formed at a period shorter than the spot diameter.
Particularly, a magnetically induced super resolution (MSR) reproduction method is proposed as a method of reproducing a recording mark smaller than a spot diameter of a beam.
In this method, a laser light for reproduction is irradiated while a magneto-optical disk in which a plurality of magnetic layers including a recording layer and a reproduction layer are stacked is rotated, so that a temperature distribution is produced in a circumferential direction of the magneto-optical disk, and a small recording mark is read out by using this temperature distribution. By this, resolution equivalent to the case where reproduction is substantially made with a light spot smaller than a spot diameter of the reproduction laser light can be obtained.
Besides, as a medium capable of reproducing recording marks recorded at a period shorter than a beam spot by using the magnetically induced super resolution, a magnetically induced super resolution medium constituted by three magnetic layers having predetermined characteristics is disclosed in Japanese Patent Unexamined Publication No. 2000-200448.
This medium is constituted by three magnetic layers of a reproduction layer, an intermediate layer, and a recording layer, and is a double mask magnetically induced super resolution medium (Double mask Rear Aperture Detection: D-RAD medium) in which a low temperature mask (called a front mask) is formed in a low temperature region of a temperature distribution formed in a beam spot, and a high temperature mask (called a rear mask) is formed in a high temperature region.
According to this medium, a recording mark having a length of 0.38 xcexcm and formed on a land substrate of a track pitch of 0.9 xcexcm can be reproduced by a reproduction magnetic field of 300 Oe or less.
Besides, in order to further increase the recording density of a recording medium, it is necessary to shorten a track pitch of the medium in a radius direction and to shorten a mark length of a recording mark.
As one method of shortening the track pitch to increase the density, there is a method in which a land groove substrate is used, and recording marks are formed on both a land and a groove.
Even if the land groove substrate is applied to the D-RAD medium, if it is a high density medium of about 2.3 GB/3.5 inches, it is possible to form a medium in which cross talk from an adjacent track is hardly generated.
However, in the case where the track pitch of the land groove substrate is shortened to further increase the capacity of a medium, there arises a problem that cross talk from an adjacent track can not be neglected and a reproduction power margin becomes narrower than a design value.
For example, in the case where a high density D-RAD medium of a track pitch of about 0.50 xcexcm is reproduced by an optical system of an LD wavelength of 660 nm and NA=0.55, the cross talk from an adjacent track is generated to such a degree that it can not be neglected, which becomes a problem at a practical use level of recording and reproduction of data.
FIG. 12 and FIG. 13 are graphs each explaining the dependency of cross talk of a conventional D-RAD medium upon reproduction power (Pr).
The vertical axis indicates a decibel value CNR (dB) equivalent to the amount of generated cross talk, and the horizontal axis indicates a reproduction power (mW).
FIG. 12 relates to a land groove substrate of a track pitch of 0.65 xcexcm, and FIG. 13 relates to a land groove substrate of a track pitch of 0.50 xcexcm.
In the graphs, in the case where 8T marks having the shortest mark length of 0.300 xcexcm are stored in a groove adjacent to a land, a decibel value of a signal leaking into the land by the cross talk is measured.
According to FIG. 12, even if the reproduction power Pr is raised from 3.6 mW to about 5.5 mW, the cross talk is 20 dB or less which does not become a problem in practical use.
Besides, when a region from Pr=3.8 mW as the rising (Prth) of CNR at which reproduction can be performed to a point at which the cross talk is 20 dB or less (to Pr=5.5 mW) is considered to be a reproduction power margin, in the case of FIG. 12, there is a reproduction power margin of xc2x118.3%.
On the other hand, in the land groove substrate of a track pitch of 0.50 xcexcm shown in FIG. 13, the cross talk exceeds 20 dB when the reproduction power Pr is approximately 4.5 mW.
That is, high cross talk is observed at the high Pr side, and the influence of the cross talk becomes high even in a region of lower reproduction power. In FIG. 13, the reproduction power margin is decreased to about xc2x19.0%.
It is conceivable that the high cross talk is observed at the high Pr side like this because of the following mechanism.
FIG. 14 is an explanatory view of magnetization states of the respective layers at the time of reproduction of a conventional D-RAD medium.
FIG. 14 shows the directions of magnetization of a reproduction layer 51, an intermediate layer 52, and a recording layer 53 as three magnetic layers of the D-RAD medium.
In a state where a reproduction magnetic field 61 is applied from below, a light beam is irradiated to the magnetic layers from above. FIG. 14 shows the magnetization states in the vicinity of a beam spot BS where the light beam is irradiated, and in the case where this medium is moved in the upper right direction in the drawing, a temperature distribution in the beam spot is divided into three regions (a low temperature region A, an intermediate temperature region B, and a high temperature region C).
With respect to the direction of magnetization, the up direction indicates a recording direction L1, and the down direction indicates an erasing direction L2.
As shown in FIG. 14, when the reproduction magnetic field 61 is applied in the recording direction L1, in the low temperature region A within the beam spot, all the magnetization of the intermediate layer 52 is directed in the external magnetic field direction L1, the magnetization of the reproduction layer 51 exchange-coupled to the intermediate layer 52 is directed in the erasing direction L2, and a low temperature mask A1 is formed in the reproduction layer 51. At this time, irrespective of the direction of magnetization of the recording layer 53 of the low temperature region, the directions of magnetization of the intermediate layer 52 are made uniform, and an interface magnetic wall 62 is produced between the magnetization of the recording layer 53 in the recording direction L1 and the intermediate layer 52.
On the other hand, in the high temperature region C within the beam spot, since the magnetization of the intermediate layer 52 reaches the Curie temperature, spontaneous magnetization disappears (expressed by a blank portion), and exchange-coupling force to the reproduction layer 51 is cut. Accordingly, in this high temperature region C, all the magnetization of the reproduction layer 51 is made uniform in the external magnetization direction L1, that is, in the reproduction layer 51, all the spontaneous magnetization is directed in the recording direction L1, and a high temperature mask C1 is formed.
Besides, in the intermediate temperature region B, a recording mark recorded in the recording layer 53 is transferred to the reproduction layer 51 by exchange-coupling through the intermediate layer 52. Here, the magnetization direction of the recording mark transferred to the reproduction layer 51 is the recording direction L1. That is, in this case, the magnetization direction in the intermediate temperature region B is directed in the same direction as the high temperature mask C1.
FIGS. 15(a) and 15(b) are views for explaining a state of generation of cross talk in the case where the D-RAD medium is seen from above.
FIG. 15(b) shows the case where a track pitch TP is relatively wide, and the high temperature mask C1 in the beam spot BS does not overlap an adjacent recording mark M1, so that the cross talk is not generated.
On the other hand, as shown in FIG. 15(a), in the case where the track pitch TP is narrower than that of FIG. 15(b), there is also a case where the high temperature mask C1 made of spontaneous magnetization directed in the recording direction L1 overlaps an adjacent recording mark M1.
It is conceivable that at this time, the recording marks M1 which must be naturally masked are connected to each other and transfer is performed to make the directions uniform in the same magnetization direction L1 as the high temperature mask C1, and this has, as a cross talk signal, an effect on the reproduction of a recording mark of an adjacent track.
On the other hand, even in the case where the mask of the high temperature region does not overlap a mark of an adjacent track, it is conceivable that the mark of the adjacent track is transferred by the following principle.
As shown in FIG. 16, from the magnetization of the reproduction layer 51 of the high temperature region C directed in the recording direction L1 by the reproduction magnetic field 61, a leakage magnetic field 63 is generated in the direction indicated by a slant arrow in the drawing. This is the same as the magnetization direction of the intermediate layer 52 in the intermediate temperature region B at the time of the transfer state, and acts in the direction to facilitate the transfer. The leakage magnetic field 63 is generated to become high in proportion to the intensity of the magnetization of the reproduction layer 51 and becomes weak in inverse proportion to the square of the distance.
In general, since the reproduction layer 51 of the D-RAD medium uses a material in which the value of saturation magnetization increases in proportion to the temperature up to about 200xc2x0 C., it is conceivable that as the mask of the high temperature region C is formed and approaches the recording mark M1 of the adjacent track, the leakage magnetic field 63 from the mask of the reproduction layer 51 of the high temperature region C acts in the direction to facilitate the transfer, and as shown in FIG. 17, it has an effect as cross talk on the adjacent track.
Like this, in the conventional D-RAD medium, when the track pitch is made narrow, the cross talk due to the high temperature mask having the spontaneous magnetization in the reproduction layer is generated, so that there is a limit in realization of high density by narrowing the track pitch.
According to the present invention, the magneto-optical recording medium includes at least three magnetic layers of a first magnetic layer, a second magnetic layer and a third magnetic layer, in which a recording mark recorded in the third magnetic layer by irradiation of a light beam is transferred through the second magnetic layer as formed above the third magnetic layer to the first magnetic layer formed above the second magnetic layer for reproduction, wherein the first magnetic layer includes a high temperature mask having no spontaneous magnetization in a region where its temperature becomes a predetermined temperature or higher.
According to this, cross talk from an adjacent track can be reduced, and recording noise and erase noise can be reduced even if a track pitch is made narrower than a conventional medium.